Inductors for use in light current electrical circuit



Sept. 22, 1959 R. C. LEIGH 2,905,913

INDUCTORS FOR USE IN LIGHT CURRENT ELECTRICAL CIRCUIT 1 Filed Dec. 7, 1955 I United States Patent Ofitice 2,905,913 Patented Sept. 22, 1959 INDUCTORS FOR USE IN LIGHT CURRENT ELECTRICAL CIRCUIT Ronald C. Leigh, Taplow, England, assignor to British Telecommunications Research Limited, Taplow, England, a British company Application December 7, 1955, Serial No. 551,634

Claims priority, application Great Britain December 14, 1954 3 Claims. (Cl. 33683) The present invention relates to inductors for use in light current electrical circuits and particularly those with arrangements for effecting some variation for trimming or similar purposes. Such inductors find considerable use in telecommunication equipment and common present-day practice is to embed or encapsulate the various component parts in solid insulating material in order to avoid trouble due to the ingress of moisture and to ensure that the inductance value shall not change owing to any movement of the parts. It is very desirable that adjustment for trimming purposes should be possible after the embedding operation, preferably by means of some small tuning member which can subsequently itself be embedded or otherwise fixed if no further adjustment is likely to be required in use. Hitherto however difficulties have been experienced in meeting these requirements readily and the object of the present invention is to enable this to be done in a simple and inexpensive manner.

More specifically an object of the invention is to provide an encapsulated inductor assembly suitable for precision circuits which assembly is incorporated in a screening can arranged to co-operate with the various individual parts to facilitate the assembly and potting operations.

The invention will be better understood from the following description of one suitable embodiment which should be taken in conjunction with the accompanying drawings comprising Figs. 1 and 2. Of these Pig. 1 shows an end view of the completed assembly while Fig. 2 is a sectional view along the line IIII.

The inductor assembly is mounted in a can 1 of metal, preferably copper, which in addition to providing mechanical protection, gives a screening effect in well-known manner. At its open end this can has secured to its sides two threaded members 2, 3 by means of which it may be secured in position on the rack or other mounting in suitable relationship to its associated circuit components. The main mounting member within the can is a moulding 4 preferably of a loaded epoxy or polyester resin provided with a boss 5 which extends through a central hole of similar size in the base of the can 1. The base of the moulding 4 is rectangular in shape so as to fit fairly closely in the can 1 and it is provided with a post 6 which is internally threaded and is open to the exterior of the can by way of the boss 5. The upper portion of the post is closed however and this portion is externally threaded for the reception of a nut 7.

The coils are mounted on a bobbin 8 which in the form shown is divided into three sections and the bobbin is enclosed within the split pot core formed of the two members 9 and 10. Though it is not apparent in the drawing, these members are provided with small semicircular openings which register in the two halves for the passage of the leads from the coils. These leads may be secured to the wires 11 which may be soldered to eyelets moulded into the base 4 at each corner. The wires also serve for effecting external connections and may have other components such as capacitors con nected thereto. The holes in the dust core for the passage of the lead-out wires may if desired be sealed with high melting point wax to prevent ingress of the casting resin.

In the assembly of the unit the coils are enclosed Within the two halves 9 and 10 of the pot core and are then slipped over the post 6 and retained in position by means of the nut 7. Suitable connections are made to the wires 11. The moulding 4 is then inserted in the can 1 and a screw with a flanged head may be inserted into the hollow post in order to maintain the assembly in position. The can is then filled with casting resin or other suitable encapsulating material as indicated at 12, the retaining screw ensuring that the material shall not escape through the hole in the end of the can. The resin is then cured or otherwise treated in the usual way to form a solid block. The retaining screw is then removed and a magnetic core 13 is screwed into the hollow post the required distance to give the appropriate value of inductance to meet the requirements of the circuit in relation to the other components. To facilitate this operation the end of the core is preferably slotted as at 14 so that it may be operated by a screw driver. When the correct value of inductance has been obtained and no subsequent change is likely to be required, the core may be locked in position by filling the threaded hole with wax or resin so that the value of inductance will not alter thereafter.

It should be mentioned that the moulding could also be made of ceramic or other suitable insulating material but the use of a loaded epoxy or polyester resin ensures good mating between the moulding and the encapsulating resin.

Moreover though the use of a screening can has been described, this may not always be necessary and the inductor would then be temporarily mounted in a mould during the encapsulating process.

According to the invention therefore the assembly of inductors with a closely controlled inductance value is considerably simplified while the advantages of potting, which is generally found desirable in certain conditions of use, are still retained.

1 claim:

1. An encapsulated inductor assembly comprising in combination, a metallic can having a hole symmetrically located in its base, a moulding of material selected from the group ceramic, loaded epoxy resin and loaded polyester resin, including a base portion slightly smaller than the cross-section of said can, a hollow post upstanding from said base and integral therewith, a hollow boss also integral with said base and projecting therefrom through said hole in said can in line with said post on the opposite side of said base, a coil forming the inductor, a split pot core of magnetic material enclosing said coil and encircling said post, means for maintaining said core in position on said post, a magnetic core within said hollow post, means accessible through said boss for varying the position of said magnetic core relative to said coil and encapsulating material filling said can and fixing the position of said post while still permitting movement of said core.

2. An encapsulated inductor assembly comprising in combination, a metallic can having a hole centrally located in its base, a moulding of loaded epoxy resin including a base portion forming a loose fit within the can, a hollow post upstanding from said base and integral therewith, said post being threaded internally and externally at its closed end, a hollow boss also integral with said base and projecting therefrom through said hole in said can in line with said post on the opposite side of said base, a coil forming the inductor, a split pot core of magnetic material enclosing said coil and encircling said post, a nut engaging with the threaded end of said post for retaining said core in position thereon, an externally threaded magnetic core within said hollow post, means accessible through said boss for rotating said magnetic core to vary its position relative to said coil and encapsulating material filling said can and fixing the position of said post while still permitting movement of said core.

3. An inductor assembly as claimed in claim 2 in which the said base portion carries a plurality of stiff wires to which the ends of said coil are connected and which project outside the encapsulating material to provide external connections.

References Cited in the file of this patent UNITED STATES PATENTS 2,461,397 Ross Feb. 8, 1949 2,668,947 Stewart Feb. 9, 1954 FOREIGN PATENTS 484,855 Great Britain May 11, 1938 

